1. Field of the Invention
This invention relates to the preparation of homopolymers and copolymers of vinyl chloride having reduced grain size and reduced porosity which are resistant to the swelling effect at room temperature of primary plasticizers for high molecular weight polyvinyl chloride.
Such fine grain homopolymers and copolymers can be obtained in the 20 micron to 50 micron average grain size by the process of the invention which have reduced susceptibility to solvation by primary plasticizers at ambient temperatures and are useful as extender resins in the preparation of plastisols and organosols. The polymers of the invention can be used in applications where previously bulk polymerized polymers were unsuited because of their excessive grain size and wide grain size distribution in addition to high plasticizer absorptivity at room temperature. Prior art bulk polymerization methods produce polymers having average grain sizes in the range of about 80 to about 150 microns. More costly suspension polymerization processes are utilized to produce fine grain size extender polymers having low primary plasticizer absorptivity.
2. Description of the Prior Art
Polyvinyl chloride is classically produced by bulk or mass polymerization methods which means polymerization or copolymerization in the absence of solvents and diluents. The polymer is produced in particulate form having irregular shape which may vary from minute specks to irregular non-globular masses. In British Pat. No. 1,047,489 and U.S. Pat. No. 3,522,227, both of which are hereby incorporated by reference, there are described a method of polymerization in mass in two steps which produces an improvement in regularity of granular size and shape. The process is more controllable and the product more uniform than can be obtained by other methods of bulk polymerization. In this process, the monomer is polymerized in a first stage to about 7 to about 15 percent conversion, preferably about 8 to about 10 percent conversion in a reactor called a prepolymerizer in which provision is made for high speed, high shear agitation. At the end of the first stage of polymerization, the mass is transferred to a second reactor and further polymerized in this second stage reactor in which provision is made for slow speed agitation to an appropriate end point (70 to 85 percent conversion in many cases).
In copending U.S. application Ser. No. 169,838, filed Aug. 6, 1971, a process is described which relates to the production of homopolymers and copolymers of vinyl chloride by a two stage polymerization method which produces a further reduction in particle size by a method involving contacting the monomer or monomers in the first stage of polymerization with an organic or inorganic, inert, particulate material solid at least at reaction temperature, a surface active agent, or a combination thereof. Examples of such solid inert materials are silicon dioxide, a hydrophobic coated silicon dioxide, solid particles obtained by emulsion polymerization of vinyl chloride, calcium carbonate, and calcium stearate. The particle size of the polymer is substantially reduced over the product of the process disclosed in U.S. 3,522,227, but nevertheless, still retains a high degree of porosity to plasticizers at room temperature. Incorporation of substantial amounts of these polymer particles as an extender resin in a plastisol can result in a rapid rise in viscosity upon ageing the plastisol at room temperature.
The polymers produced by the method of this invention are characterized by having a small particle size as obtained by the method described in copending U.S. Ser. No. 169,838, but additionally have the characteristics of lower fusion temperature and remaining substantially unsolvated when placed in contact with primary type plasticizers for vinyl chloride resins at room temperature as judged by viscosity stability tests on plastisols containing the polymer particles of the invention.